It is common practice to fracture an underground formation surrounding such a well by pumping a high pressure fluid into an area of the well which is hydraulically isolated from other parts of the well by a pair of isolation packers. The hydraulic pressure exerted to the formation surrounding that area will then initiate fractures in the formation surrounding the well. These fractures may serve to enhance inflow of oil and/or gas into the well, in which case a proppant and/or treatment fluid may be injected into the fractures to further stimulate the oil and/or gas production. Alternatively the fractures may serve to discharge drill cuttings and/or fluids into the formation.
Sometimes an inflatable sleeve is inflated in the borehole to limit loss of fracturing fluid into the fractures. The use of such a sleeve is known from U.S. Pat. Nos. 2,798,557, 2,848,052, 4,968,100, 4,657,306, 5,295,393 and 3,062,294.
Said U.S. Pat. No. 3,062,294 discloses that the expandable sleeve may be equipped with bit members which are mounted on pistons that are embedded in the sleeve and which are pushed radially into the formation to cleave the surrounding formation. The orientation of the cleaved fractures is essentially dictated by formation stresses so that the fractures are generally not parallel to the borehole.
U.S. Pat. No. 5,511,615 discloses a tool for measuring the in-situ borehole stress which tool comprises three short cylinder sections which are arranged in a vertical stack. Each cylinder section comprises two cylinder halves which are pressed against the formation to initiate a fracture generally in a plane that divides the cylinder halves. The cylinder sections are stacked in a vertically offset manner such that the planes that divide the cylinder halves of adjacent sections intersect each other at about 60 degrees. In this manner an accurate determination of the size and orientation of formation stresses can be made.
U.S. Pat. No.'s 5,678,088 and 5,576,488 disclose other mechanical fracturing tools for measuring formation stresses by temporarily creating fractures in a selected orientation into the formation, which fractures are allowed to close again after the measurement has been made.
U.S. Pat. No. 2,687,179 discloses a mechanical formation fracturing tool which comprises a pair of semitubular expansion members which are pressed in diametrically opposite directions against the borehole wall by hammering a wedge between the expansion members. The known tool is able to obtain at least partial control of the direction of fracturing but has the disadvantage that the impacts generated by the hammering action may damage the borehole wall and crush the surrounding formation in the vicinity of wellbore which reduces the control of the fracturing process. French patent specification 1602480 discloses a fracturing tool where a pair of semi-tubular elements are expanded by hydraulic pressure.
It is an object of the present invention to provide a tool and method for fracturing an underground formation where the generated fractures can be held open over a sufficient period of time to allow the placement of a proppant and a treatment or other fluid in the fracture, while causing less interruption of other activities in the borehole than would occur with the known fracturing techniques.